Japan Steel Works Muroran Plant

The Japan Steel Works, Ltd. (株式会社日本製鋼所) was established in Muroran, Hokkaido in November 1907, as a joint venture between the Hokkaido Colliery and Steamship Company (北海道炭礦汽船) and the British armaments firms Armstrong Whitworth and Vickers. Its founding purpose was to manufacture gun barrels and heavy ordnance for the Imperial Japanese Navy, reducing Japan's dependence on imported armaments.

From the 1910s through World War II, the Muroran Plant was one of Japan's principal makers of large-calibre ordnance. Its best-known products include the 41 cm main gun barrels of the battleship Mutsu and a 41 cm howitzer barrel built experimentally for the Army, the largest of its kind in Japan. The plant's massive forging presses and precision boring equipment were among the few facilities in the country capable of producing ordnance of this scale. It is often said that Muroran also made the 46 cm (18.1-inch) main guns of the Yamato-class superbattleships, but this is mistaken: all of those guns were manufactured at Kure Naval Arsenal.

Beyond these, the Muroran Plant produced a wide range of naval ordnance including 36 cm and 20 cm gun barrels, armor plate, and steel castings for warship construction throughout the war.

On July 15, 1945, the plant became a target of one of the final Allied naval bombardments of the Japanese Home Islands. Task Unit 34.8.2, commanded by Rear Admiral Oscar C. Badger and including the fast battleships USS Iowa (BB-61), USS Missouri (BB-63), and USS Wisconsin (BB-64), the light cruisers USS Atlanta (CL-104) and USS Dayton (CL-105), and eight destroyers, opened fire on the Japan Steel Works and the neighboring Wanishi Ironworks at 09:36. The bombardment lasted approximately 50 minutes, with 860 sixteen-inch shells fired at ranges of 28,000–32,000 yards. Admiral William F. Halsey Jr. accompanied the force aboard Missouri, and the sounds of the bombardment were broadcast live on American radio via a relay on Iowa.

The Japan Steel Works attempted to conceal its facilities by igniting oil to create a smoke screen. American aerial spotters mistook the smoke for damage from their shells and redirected fire toward the adjacent Wanishi Ironworks. Of the shells directed at Japan Steel Works, 64 struck within the factory grounds and 130 hit the company housing area. The bombardment killed 112 Japan Steel Works employees and injured 88 others, while deaths across Muroran are put at 427 by the US Strategic Bombing Survey and at 485 in Japanese compilations.

The plant survived the war and continues to operate today as a major industrial facility, now known for manufacturing ultra-large steel components for nuclear reactor pressure vessels. The factory site at Nakamachi, Muroran retains its industrial character from the wartime period.